The present invention relates to a method of manufacturing a combined sensor used for measurement of acceleration and angular rate.
A variety of types of capacitive physical sensors have hitherto been presented. In the physical sensor, movable mechanical components such as a vibration device and a movable device are mounted on a silicon substrate or a glass substrate through micromachining technology, drive gaps are formed in a cap substrate at locations corresponding to the movable mechanical components such as the vibration device and the movable device, and these substrates are sealed to each other by bonding or with adhesion. The movable mechanical component has a size of μm order, raising a problem that air resistance and the like factor have influence upon characteristics of the movable mechanical component to degrade them, and therefore, sensing parts must be sealed in pressure ambiences corresponding to the respective vibration device and movable device of the movable mechanical components.
Since acceleration sensors and gyroscopes are mounted on the same substrate in the combined sensor, these sensors are sealed in pressure ambiences in which characteristics of the respective sensors are not degraded. In general, a sensing part of the acceleration sensors are sealed in atmospheric pressure ambient (atmospheric pressure on sealing) and a sensing part of the gyroscope is sealed in vacuum pressure ambience (vacuum pressure on sealing) to ensure that a combined sensor prevented from being degraded in its characteristics can be provided.
The movable mechanical component of gyroscope is a vibration device and when an angular rate is applied to the vibration device being driven to vibrate at a constant frequency, Coriolis force is generated. Being applied with the Coriolis force, the vibration device is displaced. By detecting an amount of displacement of the vibration device due to the Coriolis force, an angular rate can be detected. The higher the driving speed of the vibration device, the more the Coriolis force increases and hence, for the purpose of improving the detection sensitivity of the gyroscope, the vibration device needs to be vibrated at a high frequency and with a large amplitude of several micron meters. The vibration device produced by micromachining is, however, set up in a minute gap and when the atmospheric pressure prevails in driving ambience, the dumping effect of air (sealing gas) is largely influential. The dumping effect has, therefore, an influence upon a vibration of the gyroscope driven at a high frequency and with a large amplitude and as a result, the detection sensitivity of the gyroscope is degraded. Accordingly, by sealing the sensing part of gyroscope in an ambience less affected by the dumping effect, that is, in vacuum ambience, a gyroscope capable of vibrating at the high frequency and with the large amplitude can be obtained.
On the other hand, the acceleration sensors are a movable device having its movable mechanical components such as a mass and a beam and when being applied with an acceleration, the movable device displaces. By detecting an amount of displacement, an acceleration can be detected. When the acceleration sensors are sealed in the same vacuum pressure ambience as that the gyroscope undergoes, the movable device of acceleration sensors are subjected to a small dumping effect to thereby generate a phenomenon that the movable device keeps oscillating and as a result, the acceleration cannot be detected with high sensitivity. Accordingly, the acceleration sensors are sealed in the atmospheric pressure ambience which exhibits a large dumping effect.
JP-A-2002-5950 discloses a publicly known combined sensor having acceleration sensors and a gyroscope in combination, according to which through holes (degassing passages) are formed in a cap substrate for sealing the acceleration sensors and the gyroscope, and after completion of sealing by exerting either the atmospheric pressure or the vacuum pressure on sealing, the through holes are filled either in the atmospheric pressure ambience or in the vacuum pressure ambience so that the acceleration sensors may be sealed in the atmospheric pressure ambience and the gyroscope may be sealed in the vacuum pressure ambience.
Further, in JP-A-2008-501535, the acceleration sensors and the gyroscope are sealed in the atmospheric pressure ambience and thereafter, a through hole is formed in the cap substrate at a location above and corresponding to the gyroscope. Subsequently, the through hole is filled through chemical vapor deposition (CVD) process, so that the gyroscope can be sealed in the pressure ambience for the CVD process, that is, in the vacuum ambience. In the method mentioned as above, the acceleration sensors can be sealed in the atmospheric pressure ambience and the gyroscope can be sealed in the vacuum pressure ambience.
In JP-A-2008-116423, adsorbents (getters) are disposed on a cap substrate for sensors used to detect physical quantities (acceleration and angular rate) in order that a noble gas and an activated gas can be sealed internally at predetermined partial pressures.
In JP-A-2008-118147, an adsorbent (getter) is heated and activated so that activated gases may all be adsorbed to the adsorbent (getter) and so, the interior of a sensor may be sealed in a noble gas pressure ambience.
Each of the aforementioned JP-A-2002-5950 and JP-A-2008-501535 discloses sensors of physical quantities in which after the sensor substrate and the cap substrate have been bonded together, the through holes are filled so that the sensors may be sealed in two kinds of pressure ambiences. Further, in the aforementioned JP-A-2008-116423, after the sensor substrate and the cap substrate have been bonded to each other, the adsorbent (getter) formed on the cap substrate is activated, thereby ensuring that the pressures the sensing parts undergo can be adjusted in accordance with partial pressures of sealed gas.
However, for the sake of improving the detection sensitivities of the acceleration sensors and the gyroscope both disposed on the same substrate, the sensing parts must be sealed in pressure ambiences corresponding to the movable device and vibration device of the respective sensors. Because of sealing on the same substrate, sealing in a pressure ambience corresponding to one of the acceleration sensors and the gyroscope is easy but in order to set up pressure ambiences corresponding to both the sensing parts, the method as described in the aforementioned JP-A-2002-5950 and JP-A-2008-501535 is employed, according to which the through holes are formed in the cap substrate which in turn is bonded to the sensor substrate and thereafter, the through holes are filled with a different material. But, the above method suffers from such a problem that depending on a shrinkage due to a difference in coefficient of linear expansion between silicon or glass and the material filling the through hole and on adhesive intimacy of silicon or glass with the material filling the through hole, leakage takes place through the interface.
Also, as described in JP-A-2008-116423, by disposing the adsorbent (getter) and activating the getter after completion of sealing in the pressure ambience corresponding to one of the sensing parts, the sensor is obtained in which sealing can be done in pressure ambiences corresponding to respective sensing parts. But, the pressures for the sensing parts are determined by the capacity for adsorption of activated gas the adsorbent (getter) has and by the partial pressures of noble gas and activated gas, giving rise to a problem that a fine pressure adjustment cannot be made.